Much of today's high-density integrated circuitry has been made possible by small-geometry CMOS (Complementary Metal Oxide Semiconductor) technology, well known to those skilled in the art of semiconductor processing. Most modern microprocessors and large-scale integrated circuits are made using small-geometry CMOS processes (2 micron line widths and below). Two of the biggest advantages of CMOS technology are: 1) its inherently high noise immunity (due to relatively large voltage differences between CMOS logic output voltages and input switching thresholds compared to other logic families), and 2) low power dissipation.
CMOS technology is particularly desirable in military and aerospace applications because of these noise and power characteristics. However, military and aerospace environments tend to be characterized by high levels of radiation, particularly gamma and X-ray radiation, and standard CMOS circuits are known to have some problems with high-radiation environments. While it is possible to construct radiation shields for CMOS military applications, this tends to add weight, expense, and complexity; requires extensive testing; and negates much of the desirability of CMOS for these applications.
Also well known to those skilled in the art is that CMOS technology is typified by logic gates constructed from complementary pairs of MOS Field Effect Transistors (FET's) (p-channel and n-channel) in a sort of a push-pull configuration where only one transistor of any given pair is "on" at a time, such that CMOS gates (ideally) draw no steady-state current. The inputs to CMOS gates, being the unloaded gates of insulated-gate FET's (MOS transistors are also known as insulated-gate field effect transistors, or IGFETs, by virtue of an insulating layer of SiO.sub.2 between the gate and the active channel area of the transistor), draw no steady-state current, either. The only currents drawn by CMOS circuitry are due to leakages and to switching currents, which result from the charging and discharging of parasitic capacitances at the time of a logic state change.
"Radiation hardness" refers to the ability of a semiconductor device to withstand radiation without alteration of its electrical characteristics. A semiconductor device is said to be radiation hardened (rad-hard), radiation tolerant, or radiation resistant if it can continue to function within specifications after exposure to a specified amount of radiation. Semiconductor devices can be damaged or destroyed by the effects of nuclear radiation from natural and man-made sources. Radiation changes the electrical properties of solid state devices, leading to possible failure of any system incorporating them.
Gamma rays, X rays, and neutron bombardment have proven to be the most harmful forms of radiation. Rad-hard devices and circuits have been developed to minimize the effects of these forces. The devices can be designed to be rad-hard, or the normal manufacturing process can be modified to produce rad-hard devices with special isolation techniques. Radiation hardening now permits systems designers to take advantage of the benefits of CMOS technology in high-performance, high-reliability products intended for applications where radiation is present.
The following sources of radiation are of particular interest in the present context: